1. Field of the Invention
The present invention relates to an electrophoretic display device in which charged electrophoretic particles in an insulating liquid are caused to migrate to make a display.
2. Related Background Art
Various types of electrophoretic display devices in which charged electrophoretic particles in an insulating liquid are caused to migrate to make a display have been proposed. Such an electrophoretic display device will hereinafter be described.
With the development of information instruments, the need of display devices of low consumption power and the thin type is increasing, and research and development of display devices are more and more made according to the need. Among others, liquid crystal display devices are actively developed as display devices that can meet such need and commercially provided. However, the current liquid crystal display devices involve problems that characters on a screen become hard to see by an angle at which the screen is seen or reflected light, and that a burden on sight caused by glaringness, low luminance of a light source, or the like is heavy. These problems are not yet sufficiently solved. For this reason, it is expected to develop reflection type display devices from the viewpoints of low consumption of power, lightening of the burden on sight, etc. These problems are not yet sufficiently solved. For this reason, it is expected to develop reflection type display devices from the viewpoints of low consumption of power, lightening of the burden on sight, etc.
An electrophoretic display device has been proposed by Paul F. Evans et al. as one of them (U.S. Pat. No. 3,612,758).
FIG. 9A illustrates an example of the construction of the electrophoretic display device. This kind of electrophoretic display device comprises a pair of substrates 1a, 1b arranged in a state spaced at a prescribed interval, an insulating liquid 2 filled between these substrates 1a and 1b, a great number of colored and charged electrophoretic particles 3 dispersed in the insulating liquid 2 and display electrodes 15a, 15b arranged at every pixel along the respective substrates 1a, 1b. Incidentally, reference numeral 7 indicates a partition wall provided between pixels to prevent colored and charged electrophoretic particles 3 from transferring to another pixel so as to maintain uniform display. In this device, colored and charged electrophoretic particles 3 are charged positively or negatively, and so they are adsorbed on any display electrode 15a or 15b according to the polarities of voltage applied to display electrodes 15a, 15b. Since insulating liquid 2 and colored and charged electrophoretic particles 3 are colored in different colors from each other, the color of the particles 3 is visually observed when colored and charged electrophoretic particles 3 are adsorbed on display electrode 15a on the side of an observer (see FIG. 9B). When colored and charged electrophoretic particles 3 are adsorbed on display electrode 15b on the other side, the color of insulating liquid 2 is visually observed (see FIG. 9A). Accordingly, the polarity of voltage applied controls every pixel, whereby various images can be displayed. The device of this type will hereinafter be referred to as xe2x80x9cvertical migration type electrophoretic display devicexe2x80x9d.
In the electrophoretic display device of such a vertical migration type, however, a coloring material such as a dye or ion must be mixed into insulating liquid 2, and the presence of such a coloring material is easy to act as an unstable factor in an electrophoretic operation because transfer of a new electric charge is brought about, and so performance, life and/or stability as a display device may have been lowered in some cases.
As those for solving such a problem, electrophoretic display devices of the type shown in FIGS. 10A and 10B (hereinafter referred to as xe2x80x9chorizontal migration type electrophoretic display devicexe2x80x9d) are disclosed in Japanese Patent Application Laid-Open Nos. 49-5598, 49-024695 and 11-202804. Such a horizontal migration type electrophoretic display device comprises a pair of substrates 1a, 1b arranged in a state spaced at a prescribed interval, an insulating liquid 2 filled between these substrates 1a and 1b, a great number of colored and charged electrophoretic particles 3 dispersed in the insulating liquid 2 and a pair of display electrodes 25a, 25b arranged at every pixel. The pair of display electrodes 25a, 25b are arranged side by side along one substrate 1b, and arranged so as not to hold insulating liquid 2 between them unlike the above-described type. In the case of such a horizontal migration type electrophoretic display device, the problem described above can be avoided because it is only necessary for the insulating liquid 2 to be transparent and there is no need to mix any coloring material. In this device, one display electrode 25a is coated with a color layer of the same color (for example, black) as that of the charged electrophoretic particles 3, and the other display electrode 25b is coated with a color layer of any other color (for example, white). The colored and charged electrophoretic particles 3 migrate horizontally (in a direction along the substrates) according to the polarities of voltage applied to display electrodes 25a, 25b and are adsorbed on display electrode 25a or 25b. When colored and charged electrophoretic particles 3 are adsorbed on display electrode 25a, the color of display electrode 25b is easier to be visually observed (see FIG. 10B). When colored and charged electrophoretic particles 3 are adsorbed on display electrode 25b, the whole pixel is visually observed with the same color as that of the charged electrophoretic particles 3 (sec FIG. 10A). Accordingly, the polarity of voltage applied controls every pixel, whereby various images can be displayed.
When it is intended to achieve higher display contrast in such an electrophoretic display device as described above, it is required to narrow the area of electrode 25a and moreover cause all the charged electrophoretic particles 3 to be adsorbed on electrode 25a. However, there has been a limitation.
In addition, the system that display is conducted by voltage, such as electrophoretic display or liquid crystal display, has involved a problem that a leakage electric field occurs between various wirings and display electrodes to cause white skip or black skip in the vicinity of the wiring of a display pixel, thereby deteriorating display quality.
The present invention has been made with the foregoing circumstances in view and thus has as its object the provision of an electrophoretic display device which can achieve higher display contrast and prevent the deterioration of display quality.
The above object can be achieved by the present invention described below.
According to the present invention, there is provided an electrophoretic display device comprising first and second substrates arranged in opposed relation to each other with a space, an insulating liquid arranged into the space and a plurality of colored and charged electrophoretic particles dispersed in the insulating liquid, in which on the basis of arrangement of a stage along the second substrate in the space between the substrates, a first surface facing a thicker portion of the insulating liquid, a second surface facing a thinner portion of the insulating liquid and side wall of the stage, which connect the first surface to the second surface, are formed on the second substrate, a first electrode is arranged along the first surface, and a second electrode is arranged along the second surface, wherein the display device has a third electrode arranged along at least a part of the side wall of the stage and comprising an electrode face continuous with the first electrode.